1. Field of the Invention
The present invention relates to emission microscopes, and more specifically to a high spatial resolution microscope using slow, highly charged ions as the excitation source.
2. Description of Related Art
The need for highly sensitive surface analytical techniques has increased, particularly due to the demands of the semiconductor industry, which routinely operates at critical dimensions on the nanometer scale. A variety of instruments and techniques are available for high resolution lateral imaging of materials, such as transmission electron microscopy, scanning electron microscopy, scanning tunneling microscopy, and atomic force microscopy. In addition, many techniques are available to determine material composition of the imaged materials, including Auger electron spectroscopy, photoelectron spectroscopy, and secondary ion mass spectroscopy. Among the currently available techniques, secondary ion mass spectroscopy (SIMS) is highly favored because it offers in-depth information, low detection limits, and high depth resolution.
Emission microscopes, which are a special class of electron microscopes, accelerate and image low energy electrons and/or other charged particles emitted or reflected from a planar sample surface. One type of conventional emission microscope is an Ar.sup.+ ion beam induced electron emission microscope, where the kinetic emission of electrons from the surface forms an image. In a second type of emission microscope, a secondary ion emission microscope, an ion beam of energetic singly charged ions is used to sputter secondary ions that are then imaged. It would be advantageous to combine the capabilities of both microscopes to image both electrons and the secondary ions, and to determine the mass of the secondary ions by time-of-flight (TOF) so that all of the secondary ions are detected simultaneously. An emission microscope for imaging and mass spectrometry to acquire spatially-resolved, compositional information with high sensitivity would have unique capabilities.
However, the use of singly charged ions for sputtering limits the technique due to the low secondary ion yield per incident ion. Most of the energy transferred to the sample surface comes from the kinetic energy of the projectile ion. Typically, the sputter yield is about 2-10 sample ions per incident ion, and the secondary ion yield per incident ion is often less than 10.sup.-2. The number of secondary ion counts per unit of sample consumption primarily determines the sensitivity limit.
The low secondary ion yield may be overcome by the use of highly charged ions for sputtering. The technique of SIMS using highly charged ions is described in a co-pending patent application (Ser. No. 09/227,997). The use of highly charged ions has many advantages, since these ions bring considerable energy to the sample surface which is released in a few femtoseconds of the surface interaction. This release causes highly localized energy deposition, which results in very high electron yields (hundreds of electrons per incident ion) and dramatically enhanced, high secondary ion yields (greater than one). The ratio of the secondary ion yield to the secondary neutral yield gives ionization probabilities of 10% for highly charged ion excitation. The high electron yield allows very high spatial resolution.
The present invention addresses the limitations of conventional emission microscopes by providing an emission microscope that images both electrons and secondary ions, and uses slow, highly charged ions as the sputtering source to achieve at least an order of magnitude greater sensitivity. The present invention provides highly sensitive compositional information at high spatial resolution.